In the recovery of oil from oil-containing formations, it usually is possible to recover only minor portions of the original oil-in-place by the so-called primary recovery method which utilizes only the natural forces present in the formation. Thus, a variety of supplemental recovery techniques have been employed in order to increase the recovery of oil from subterranean formations. The techniques include waterflooding, miscible flooding, and thermal recovery methods.
A problem which arises in the various flooding processes is that different strata or zones in the reservoir often possess different permeabilities so that displacing fluids enter the high permeability or "thief" zones in preference to zones of lower permeability where significant quantities of oil may be left unless measures are taken to plug the high permeability zones wholly or partly and so divert the displacing fluid into the low permeability zones. Mechanical isolation of the thief zones has been tried, but vertical communication among reservoir strata often renders such measures ineffective. Physical plugging of the high permeability zones by cements and solid slurries has also been attempted with varying degrees of success, but here the most serious drawback is the possibility of permanently closing still productive horizons.
From these early experiences, the desirability of designing a viscous slug capable of sealing off the most permeable layers so that the floodwater would be diverted to the underswept, tighter regions of the reservoir, became evident. This led to the use of various materials for controlling the permeability of the formations in a process frequently referred to as "profile control", a reference to the control of the vertical permeability profile of the reservoir. Profile control agents which have been proposed have included oil/water emulsions, gels, e.g., lignosulfonate gels and polymers, with polymers being the most extensively applied in recent years.
Among the polymers so far examined for improving waterflood conformance are polyacrylamides, polysaccharides, celluloses, furfural-alcohol and acrylic/epoxy resins, silicates and polyisocyanurates. A major part of this work has been conducted with the polyacrylamides. Polyacrylamides have been used both in their normal, noncrosslinked form as well as in the form of cross-linked metal complexes, as described, for example, in U.S. Pat. Nos. 4,009,755; 4,069,869 and 4,413,680. In either form the beneficial effects derived from these polyacrylamides seem to dissipate rapidly due to shear degradation during injection and sensitivity to reservoir brines.
Another group of polymeric thickeners which has received considerable attention for use in waterflooding is the polysaccharides, particularly the xanthan polysaccharides, that is, the polysaccharides produced by the action of bacteria of the genus Xanthomonas on carbohydrates. For example, U.S. Pat. Nos. 3,757,863 and 3,383,307 disclose mobility control by the use of polysaccharides in the presence of polyvalent metal ion crosslinking agents. U.S. Pat. No. 3,810,882 discloses the possibility of using certain reducible complex metal ions as cross-linking agents for polysaccharides. U.S. Pat. Nos. 4,078,607 and 4,104,193 describe a method for improving the efficiency of waterflooding operations by a particular polysaccharide prehydration technique. U.S. Pat. No. 4,413,680 describes the use of cross-linked polysaccharides for selective permeability control in oil reservoirs.
U.S. Pat. No. 3,908,760 describes a polymer waterflooding process in which a gelled, water-soluble Xanthomonas polysaccharide is injected into a stratified reservoir to form a slug, band or front of gel extending vertically across both high permeability and low permeability strata. This patent also suggests the use of complexed polysaccharides to block natural or man-made fractures in formations. The use of polyvalent metal ions for cross-linking polysaccharides and other polymers which are to be used for permeability control is described in U.S. Pat. Nos. 4,009,755, 4,069,869 and 4,413,680.
The use of either organic or inorganic polymers in the gel form is especially desirable for profile control because the polymer is a relatively minor constituent of the gel, with water filling the majority of the pore space in the formation. The range of polymer concentration needed to form a stable gel can be as low as 0.2% for a chromium cross-linked biopolymer to 4-10% for polyacrylamides cross-linked with methylene-bisacrylamide. Therefore, what is needed is a method which can further decrease the amount of polymer utilized in a profile control treatment that would make profile control treatments still more effective and economical.